1. Field of the Invention
The present invention is generally related to a positive photoresist that can be used as the top imaging layer in a bilayer scheme or as the imaging layer in a single layer scheme, and, more particularly, to a positive photoresist for deep ultraviolet radiation (DUV), x-ray, or e-beam lithography that exhibits high sensitivity and high resolution and can be developed with metal-ion-free aqueous alkali.
2. Description of the Prior Art
The increasing density of integrated circuits has created a need for higher resolution patterning capabilities. One method of improving resolution involves using a shorter wavelength light during pattern formation. Shorter wavelengths of approximately 200-280 nm may be obtained by using a DUV source such as a Hg/Xe lamp with the appropriate filters. Additionally, KrF (248 nm) or ArF (193 nm) excimer lasers may be used as exposure sources. However, at shorter wavelengths the depth of focus of the exposure tool, which may be an excimer stepper, or step and scan tool, may be adversely affected. The depth of focus (DOF) is an expression of the range of distances from the image focal plane through which the projected image remains in subjectively acceptable focus. DOF is related to wavelength and lens numerical aperture according to the formula: EQU DOF .alpha..lambda./2(NA).sup.2
where .lambda. is the wavelength of exposing light and NA is the numerical aperture of the lens. Generally, a depth of focus of 1 to 2 .mu.m is required for an adequate lithographic process window, in order to accommodate variations in the thickness or height of the resist film.
In addition to using shorter wavelengths during exposure, it is also desirable to use a thinner layer of resist. However, as shown in FIG. 1, the major drawback of using a thin layer of resist 27 is that the variation of resist thickness over a diffusion step 21 on a substrate 23 and into an etched pattern 25 increases as the pattern size becomes smaller. This variation means that the dimensions of any pattern being imaged in the resist will vary as the step geometry is traversed. Therefore, in a single layer resist system, the lack of dimensional control on the wafer can create different line widths throughout the resist which reduces the quality of the electronic package.
To improve dimensional control, bilevel or multilevel resist systems are often utilized. A typical bilevel system is shown in FIG. 2. A bottom resist 1 is first applied to the substrate 3 to planarize wafer topography. The bottom resist 1 is cured. A second thinner imaging top resist 5 is then applied over the bottom resist 1. The top resist 5 is soft baked and a pattern 7 formed using conventional resist exposure and development, followed by etch transfer of the top pattern through the bottom resist 1 using the top resist pattern 7 as an etch mask.
Positive resists are commonly used in bilayer or multilayer applications and are usually based on novolac resins, which are the condensation polymers of substituted phenols and formaldehyde. Positive resists containing a photosensitive material become soluble in the areas which are exposed to radiation. The developers for positive resists either contain metallic ions, typically sodium or potassium, or are classified as metal-ion-free developers, which primarily contain tetramethyl ammonium hydroxide (TMAH). The metal-ion-free developers are becoming more widely used due to the cleaner processing environment. More specifically, the silicate or borate salts of metal-ion developers can cause particulate contamination around the processing equipment when residue dries. Further, the sodium or potassium can contaminate oxide films with mobile ions.
Sugiyama et al., Positive Excimer Laser Resists Prepared with Aliphatic Diazoketones, Soc. Plastics Eng., Conference Proceedings 51-60 (Nov. 1988), disclose a new class of alkali-developable positive excimer laser resists. The two-component resists are designed for DUV lithography and are comprised of .alpha.-diazoacetoacetates blended with polyhydroxybenzylsilsesquioxane (PHBS) as a matrix resin.
U.S. Pat. No. 4,745,169 discloses silicon-containing polymers for use in bilayer resist applications. The base soluble silsesquioxane polymer is synthesized by reacting trimethylsilyl iodide with polymethoxybenzyl-silsesquioxane to form aryl-o-trimethyl silyl groups. These trimethyl silyl groups are then hydrolyzed in water to form hydroxy groups. However, this reaction is not highly reproducible and often gives crosslinked polymer. Moreover, when these polymers are combined with diazonaphthoquinone-based photoactive compounds (PACs), exposure doses of &gt;100 mJ/cm.sup.2 at 365 nm are required to pattern the resist. Resists containing such (PACs) are too optically dense in the 200-280 nm region to be practical for DUV lithography. The optical density is greater than 0.5 for a 0.3 .mu.m film of 20% PAC in any polymer and the imaging dose is greater than 50 mJ/cm.sup.2 in the DUV range. The optical density should be less than 0.3 or 0.4 .mu.m for a single layer resist film in order to provide the most vertical wall profiles. For thinner films in a bilayer system the optical density should typically be less than or equal to 0.3 for a 0.3 to 0.4 .mu.m film.
Japanese Patent No. JP 63-241542 uses chemical amplification in combination with dissolution inhibition to provide increased processing rates using lower exposure doses. Generally, during exposure in a non-chemically amplified system, absorption of 1 photon by the photoactive compound generates 1 molecule of a carboxylic acid. However, in a chemically amplified system, 1 molecule of a sulfonic acid will further react with acid labile groups, thereby allowing the removal of up to 1000 acid labile groups with the absorption of a single photon; this, in turn, provides an increased rate of solubilization in the exposed areas of the resist. Thus, it is possible to utilize lower exposure doses in a chemically amplified system. In the method disclosed in the Japanese patent, polysilsesquioxane is first acetylated and then carboxylic acid or hydroxyl groups are generated. Poly(t-butyldimethylsilyloxystyrene), poly(butyloxycarbonyloxystyrene) or other polymers with pendant groups that can undergo a polarity change catalyzed by strong acid must be added in a ratio of about 0.1/1 to make the polysilsesquioxane insoluble in base developers. Onium salts are used as photoacid generators. Onium salts are sometimes undesirable, however, because they have limited solubility in some solvents and can have a substantial absorbance in the 240-254 nm range.